1. Field of the Invention
The present invention relates to lithium-sulfur batteries, and more specifically, to an electrolyte for use in a lithium-sulfur battery having excellent electrochemical properties such as battery capacity, high rate performance, cycle life, and performance at a low temperature.
2. Description of the Related Art
The development of portable electronic devices has led to a corresponding increase in the demand for secondary batteries having both a lighter weight and a higher capacity. To satisfy these demands, the most promising approach is a lithium-sulfur battery with a positive electrode made of sulfur-based compounds.
Lithium-sulfur batteries use sulfur-based compounds with sulfur-sulfur bonds as a positive active material, and a lithium metal or a carbon-based compound as a negative active material. The carbon-based compound is one which can reversibly intercalate or deintercalate metal ions, such as lithium ions. Upon discharging (i.e., electrochemical reduction), the sulfur-sulfur bonds are cleaved, resulting in a decrease in the oxidation number of sulfur (S). Upon recharging (i.e., electrochemical oxidation), the sulfur-sulfur bonds are reformed, resulting in an increase in the oxidation number of the S. The electrical energy is stored in the battery as the chemical energy during charging, and it is converted back to electrical energy during discharging.
With respect to specific density, the lithium-sulfur battery is the most attractive among the currently developing batteries since lithium has a 3,830 mAh/g of specific capacity, and sulfur has a 1,675 mAh/g of specific capacity. Further, the sulfur-based compounds are less costly than other materials and are environmentally friendly.
Nevertheless, no lithium-sulfur batteries have yet been made widely commercially available up to now. One reason these batteries have not been able to be commercialized thus far is due to the poor sulfur utilization over repeated cycling, resulting in a low capacity. The sulfur utilization is referred to as a ratio of the amount of the sulfur involved in the electrochemical redox reaction of batteries to the amount of total injected sulfur. Further, the sulfur is diffused away to electrolytes upon the redox reaction so as to deteriorate the cycle life characteristics. Accordingly, unless the electrolyte is suitable, the reduced product of the sulfur, lithium sulfide (Li2S), is deposited and, as a result, does not participate in further electrochemical reactions.
U.S. Pat. No. 6,030,720 describes liquid electrolyte solvents including a main solvent having the general formula R1(CH2CH2O)nR2, where n ranges between 2 and 10, R1 and R2 are different or identical alkyl or alkoxy groups, and having a donor solvent of a donor number of 15 or more. Further, it includes a liquid electrolyte solvent including a solvent having at least one of a crown ether, a cryptand, and a donor solvent, which are solvents generating a catholyte after discharging. Despite using this kind of electrolyte, however, lithium-sulfur batteries have failed to obtain satisfactory capacity, high rate performance, or cycle life characteristics.
According to current research, an electrolyte of salts and an organic solvent are anticipated to provide lithium ion batteries with a high ion conductivity and a high oxidation potential. In such lithium ion batteries, lithium salts such as LiClO4, LiBF4, or LiPF6 are mainly used. U.S. Pat. No. 5,827,602 describes non-aqueous batteries having lithium salts comprising triflate, imide, or methide-based anions. The aforementioned electrolyte shows good performance for lithium ion batteries. However, in lithium-sulfur batteries, the electrolyte causes problems by deteriorating the battery performance. This deterioration is due to the electrochemical reaction of the polysulfide being very unstable in a carbonate-based electrolyte, which is the most commonly used electrolyte in lithium-ion batteries. Thus, the lithium-sulfur batteries cannot effectively use the electrolyte present in the lithium-ion batteries. The electrolyte usable in lithium-sulfur batteries needs to have a stable electrochemical reaction with the polysulfide and needs to have the highly concentrated polysulfide generated by the reaction to be dissolvable.
Recently, attention has been drawn to a liquid-phase imidazolium cation-based salt usable at room temperature, commercially available as IONIC LIQUIDS. These cation-based salts are non-aqueous electrolyte salts capable of being applied to an electrical storage device such as a high-capacity capacitor or a battery (Koch, et al., J. Electrochem. Soc., Vol. 143, p 155, 1996). As disclosed in U.S. Pat. No. 5,965,054, a non-aqueous electrolyte containing a liquid salt such as 1-ethyl-3-methylimidazolium hexafluorophosphate (EMIPF6) is useful, having a high conductivity (>13 mS/cm), a large window of electrochemical stability (>2.5 V), a high salt concentration (>1M), high thermal stability (>100° C.), and a large capacitance (>100 F/g) from activated carbon electrode, in a double-layer capacitor.
Further, U.S. Pat. No. 5,965,054 discloses a liquid salt and an electrolyte in which the liquid salt is mixed with various carbonate-based organic solvents (J. Electrochem. Soc. Vol. 146. p1687, 1999). The electrolyte shows improved characteristics, such as a high ion conductivity (>60 mS/cm), a large window of electrochemical stability (>4 V at 20 uA/cm2), and a higher salt concentration (>3M). U.S. Pat. No. 5,973,913 discloses that, when the electrical storage devices such as an electrochemical capacitor or a battery have used the electrolytes including the above-mentioned liquid salts, they have improved characteristics such as a high capacitance and a high energy density.
However, despite the fact that the battery performance depends upon the kind and composition of the salt and organic solvent used in the electrolytes, none of the above-mentioned patents and articles concretely disclose an optimum kind and composition of salts and organic solvents for lithium-sulfur batteries, where the salts provide a high capacity, an excellent high rate performance, and a good performance at a low temperature. Particularly, lithium-sulfur batteries having liquid salts have thus far not been developed.